@katrina123 , I had a mastectomy 4 yrs ago. Following the surgery my right side ribcage area was sore to lay on. Like I was laying on something and it was pushing on my ribs. I thought it was from surgery and would get better. Fast forward - my shoulder blade had terrible spasms that would cause me 7-9 level pain. I would work thru these, chiropractor & massage. Finally PT & acupuncture. The acupuncturist identified it as scar tissue and PT therapist agreed. My Serratus Anterior muscle was affected.
Long story short, I have been working on my posture, doing acupuncture for a year now (alleviated muscle spasms) & chiro is all helping. I believe that the acupuncture has made the difference along with me doing exercises to work the serratus anterior muscle.
I know you had a lumpectomy, but food for thought, acupuncture may help you. Also Heat on the affected area. Good luck & Blessings